1. Field of the Invention
The present invention relates to an electroacoustic transducer which is used for a mobile phone, etc.
2. Description of the Related Art
FIG. 6 is a sectional view of an electroacoustic transducer of prior art. A core 3 having a coil 2 wound thereon is fixed to a base 4 at the bottom. The base 4 is fixed to a case 5 made of synthetic resin. A diaphragm 6 is disposed in the case 5 and a cover 7 is laid over the case 5. An annular magnet 8 is disposed around the coil 2 and fixed to the base 4. A magnetic flux guide member 9 made of magnetic material is disposed between the magnet 8 and the diaphragm 6. The magnetic flux guide member 9 reduces magneto-resistance of magnetic flux running through the magnet 8, the magnetic flux guide member 9, the diaphragm 6, the core 3, and the base 4, thus increasing electromagnetic force for driving the diaphragm 6. Such magnetic flux guide member is disclosed in a prior art such as JP-B-3149412. In the prior art, the diaphragm 6 is supported by the case 5, and the magnetic flux guide member 9 is directly fixed on the top of the magnet 8 by adhesive. As an alternative, the magnetic flux guide member 9 is fixed to the magnet 8 by insert molding.
To prevent a variation in sound characteristics of every product, it is highly important to strictly control the distance g1 between the magnetic flux guide member 9 and the diaphragm 6 in a state of unenergization of the coil 2. In the prior art, however, the magnetic flux guide member 9 is fixed to the magnet 8 while the diaphragm 6 is supported by the case 5. The distance g1 is therefore influenced by the height of the magnet 8 and also the height of a diaphragm supporting portion of the case 5. Thus, a strict control of the distance g1 is hardly available due to accumulation of dimensional tolerance of the magnet 8 and the case 5.
Further, when adhesive is used as in the prior art, the amount of adhesive needs to be maintained constant since it otherwise directly affects the distance g1. Generally, a magnet having higher energy product BHmax such as a sintered magnet is used in an electroacoustic transducer of this kind in order to secure enough amount of magnetic flux in a state of unenergization of the coil 2. The sintered magnet, however, inevitably requires a post process such as grinding to obtain dimensional accuracy of the magnet 8. When adhesive is used to fix the magnetic flux guide member 9 to the top of the magnet 8, the manufacturing process requires the adhering process, the strict amount control of adhesive, and the post process for the magnet. This increases the cost.
When insert molding is adopted as in the prior art, the magnetic flux guide member 9 is precisely positioned with respect to the magnet 8 by a mold. The distance g1 is therefore maintained with accuracy without a troublesome manufacturing process. The magnet 8, however, must be made of injection molding material for the purpose of insert molding. A plastic magnet is typically used though it has a lower energy product BHmax than the sintered magnet. Then the magnet volume needs be increased to secure enough amount of magnetic flux.
As the magnet volume is increased, the back space of the diaphragm 6 is reduced, affecting the operation of the diaphragm 6 by air dumper effect to lower the sound pressure level. If the back space of the diaphragm 6 is maintained, the overall dimensions of the electroacoustic transducer is inevitably increased. Further, insert molding is a costly alternative since it requires a dedicated equipment and then increases a price of a component.
Since the magnetic flux guide member 9 is located immediately below the diaphragm 6, it blocks the passage of air when the diaphragm 6 is operated in a state of energization of the coil 2. It adversely affects the operation of the diaphragm 6 by air dumper effect to lower the sound pressure level. A through-hole may be formed in the magnetic flux guide member 9 to communicate a space 12 formed between the diaphragm 6 and the magnetic flux guide member 9 with a space 11 formed inside the magnet 8. In the prior art, however, the magnetic flux guide member 9 is directly fixed on the top of the magnet 8, the magnet 8 also needs to have a notch or an opening formed corresponding to the through-hole formed in the magnetic flux guide member 9. Such irregular shape of the magnet 8 would prevent an even flow of magnetic flux to the diaphragm 6; thus possibly causing an abnormal operation of the diaphragm 6. Further, the sintered magnet or a magnet having higher energy product BHmax is difficult to be shaped to have a notch or an opening.
An object of the present invention is to provide a downsized and low-cost electroacoustic transducer having higher reliability eliminating a variation in sound characteristics for every product.
The present invention relates to an electroacoustic transducer comprising:
a base;
a core fixed upright on the base;
a magnet disposed around the core;
a diaphragm supporting member made of non-magnetic material and disposed around the magnet;
a diaphragm supported on the diaphragm supporting member; and
a magnetic flux guide member made of magnetic material, disposed between the magnet and the diaphragm, and fixed to the diaphragm supporting member.
According to the present invention, the diaphragm supporting member commonly has the diaphragm and the magnetic flux guide member fixed thereon. Since the diaphragm and the magnetic flux guide member are supported on the common member, the distance between the diaphragm and the magnetic flux guide member in a state of unenergization of a coil is easily controlled to a predetermined value with accuracy. Therefore, a variation in sound characteristics for every product is prevented and product reliability is improved. Since the magnet is free from the magnetic flux guide member, a magnet having higher energy product BHmax is available to secure enough magnetic flux at low cost.
Further, in the electroacoustic transducer of the present invention, the diaphragm supporting member has a projection and the magnetic flux guide member has an opening or a notch. The magnetic flux guide member is positioned with respect to the diaphragm supporting member by engagement of the opening or a notch with the projection. The magnetic flux guide member is fixed to the diaphragm supporting member by plastic deformation of the projection.
According to the invention, the magnetic flux guide member is restricted in position on the diaphragm supporting member. This structure easily achieves positioning accuracy of the magnetic flux guide member with respect to the diaphragm supporting member. Further, since the projection of the diaphragm supporting member is subject to plastic deformation, fixing strength by mechanical connection is so enough that no diaplacement or dropout of the magnetic flux guide member due to external shock would occur.
Further, in the electroacoustic transducer of the present invention, a gap for the passage of air is formed between the magnet and the magnetic flux guide member, and the magnetic flux guide member has an opening or a notch for the passage of air.
Since such gap for the passage of air is formed between the magnet and the magnetic flux guide member, air dumper effect is reduced without modifying the magnet in shape, thus preventing an abnormal operation of the magnet and improving product quality.